We have studied the active site structure of oxidized and dithionite-reduced formate dehydrogenase from Desulfovibrio desulfuricans using XAS at both the Mo and Se K-edges. From the molybdenum perspective the oxidized Mo(VI) active site has four Mo-S ligands (consitant with a bis-pterindithiolene cofactor coordination), with one Mo=O and one Mo-Se, while the reduced Mo(IV) active site contains four Mo-S, one Mo-O (-OH or -OH2) and one Mo-Se. From the Se perspective, the oxidized protein has, in addition to the Se-Mo and Se-C ligands expected, a Se-S with a bond-length of 2.19 E. In reduced protein this Se-S is not present. This coordination is similar to that observed for E. coli formate dehydrogenase (G. N. George, C. M. Colangelo, J. Dong, R. A. Scott, S. V. Khangulov, V. N. Gladyshev, T. C. Stadtman J. Amer. Chem. Soc. 1998, accepted for publication), except that in the D. desulfuricans enzyme dithionite can reduce the Se-S bond.